U.S. Pat. Nos. 6,335,818, 6,362,915 and 6,496,298 are assigned to the same assignee of the present application. The entire disclosures of these patents and others that are cited in the REFERENCES below are incorporated herein by reference in their entirety.
Disclosed are additives used in making well-behaved bichromal elements in display media, such as Gyricon displays, which are known. Bichromal elements, which are spherical in shape and of about 50 microns in diameter, perform the function of pixels in images formed in these displays, as described further below. Usually, the images suffer from poor quality because the bichromal balls having oppositely colored hemispheres with opposite charges do not assume proper positions; that is, they do not rotate in a precise manner, so as to make the images sharp and clear. What is needed, therefore, is a means for making the bichromal balls behave well and predictably. The charge control agents (CCAs) that are disclosed herein are used as an additive during compounding of black and white waxes that are components of bichromal Gyricon balls to provide better hemispherical control of charges, and hence improved performance under varied electric fields to yield enhanced images on especially Gyricon displays.
Display media, such as Electric Paper or twisted ball panel display devices, are known and are described, for example, in REFERENCES given below and are incorporated herein by reference in their entirety. The media generally are comprised of an encapsulant medium material, for example, an elastomer, such as a cured polysiloxane, sandwiched between two indium tin oxide coated substrates, such as glass or Mylar™. Generally, the elastomer layer has closely packed cavities, each containing a bichromal sphere suspended in a dielectric liquid. The dielectric liquid may also be present in substantial amounts in the elastomer matrix. In media that are active in an electric field, the bichromal spheres have a net dipole due to different levels of charge on the two sides of the sphere. An image is formed by the application of an electric field to each pixel of the display, which rotates the bichromal spheres to expose one color or the other to the viewing surface of the media. The spheres may also have a net charge, in which case they will translate in the electric field as well as rotate. When the electric field is reduced or eliminated, the spheres ideally do not rotate further; hence, both colors of the image remain intact. This image bistability is one feature of display media made with bichromal Gyricon balls.
The composition of certain bichromal spheres is known, for example, as set forth in the references given below. The spheres comprise black polyethylene with a light reflective material, for example, titanium oxide, sputtered on one hemisphere. In a different approach, a rotary ball is prepared by coating white glass balls of about 50 microns in diameter, with an inorganic coloring layer such as indium by evaporation. In a similar process, bichromal balls comprising glass balls are first heavily loaded with a white pigment such as titanium oxide, followed by coating from one direction in a vacuum evaporation chamber with a dense layer of nonconductive black material which coats only one hemisphere.
Methods and apparatus for fabricating bichromal elements are known and referenced below. One apparatus comprises a separator member having opposing first and second surfaces and an edge region in contact with both surfaces, and delivery means for flowing first and second colored hardenable liquid material over the first and second surfaces, respectively, so that the liquid materials arrive at the edge, usually at substantially the same flow rate, and form a reservoir outboard of the edge region. The reservoir comprises side-by-side regions of different colors, which in a preferred embodiment, do not intermix. Further means are provided for propelling the first and second liquid materials away from the separator member and out of the reservoir into a fluid medium. As this occurs, a plurality of forward ends of side-by-side bichromal streams become unstable and break up into droplets. The droplets form into spherical balls, each of the balls approximately comprising hemispheres of differently colored hardenable liquids. These bichromal balls are from about 5 to about 200 microns in diameter.
The aforementioned display media can suffer from drawbacks caused by incomplete or lack of rotation of the bichromal balls. When the balls do not rotate close to 180 degree, the switching from one color to the other is not complete. As a result, image quality suffers. In some cases, increasing the strength of the electric field used to rotate the spheres can help in achieving more complete rotation, but in other cases sufficient rotation cannot be attained, even at higher fields. In the latter cases, it is believed that the dipole strength of the sphere relative to the monopole strength is too small, rendering it difficult to get sufficient rotation before the sphere translates across its cavity in the elastomer matrix. Many of the balls lack sufficient monopole and dipole strengths to dislodge them from the cavity walls.
Materials that can improve the rotational behavior of bichromal balls could enable display media to be used in a wider variety of applications than is currently possible. For example, materials that provide a more reproducible and lower voltage for rotation and a sharper voltage threshold above a certain value can be used to make bichromal passive matrix displays in which balls that do not experience a sharp threshold voltage do not alter their state. Therefore, it is desirable to provide a display media wherein a threshold voltage of a particular value exists. It is further desirable to provide a display media wherein the threshold voltage is sharper to eliminate most, or ideally all, of the rotation below the threshold voltage and more complete rotation can be obtained at a lower applied voltage. It is still further desirable to provide a display media in which the balls have sufficient monopole and dipole strengths to allow the electric field not only to pull them from the cavity walls and translate, but also to rotate those 180° completely.
Disclosed herein are additives used in making well-behaved bichromal elements in display media. The additives comprise charge control agents (CCAs) that enable lower switching voltages, faster and more complete rotation of balls, less stiction on the cavity walls, and more distinct voltage thresholds for displays and display media containing bichromal spheres or balls, and in particular, Gyricon balls. The bichromal ball formulation disclosed herein provides improvingly stronger monopole and dipole strengths.